This invention relates generally to thermal inkjet pen construction, and more particularly to a thermal inkjet pen having a backpressure sensor.
The component within an inkjet printer that places ink onto a media sheet is referred to as an inkjet pen. The inkjet pen includes a printhead through which ink is ejected. The pen also includes ink channels for flowing ink from an ink supply to the printhead. For some pen types the ink supply is part of the pen and is stored in a local reservoir within the pen body. In other pen types, the pen may have an ink supply that continuously moves ink. For example, the pen may be a recirculating ink pen that continuously moves ink from the external ink supply into the pen then back to the ink supply. Alternatively the pen may receive ink into a local reservoir on the pen body from an external ink source.
The inkjet printhead includes one or more printhead dies and may be an elongated printhead, such as for a page-wide array printhead, or a short printhead. The printhead may be stationary as in a pagewide array design or scanned as in a short printhead. An elongated printhead less than a page width also may be scanned.
Each printhead die includes a number of nozzles through which ink drops are selectively expelled, and an ink feed slot. Some printhead dies also include an ink manifold (channels) coupling the nozzles to the ink slot. Each nozzle includes a nozzle chamber, a firing element, ink feed channels or openings, and a nozzle orifice.
Typically, ink flows from the ink feed slot through the ink feed channels into the nozzle chamber under capillary action. Specifically, the geometry of the chamber and channel allow ink to be drawn from the ink slot in response to a nozzle firing. The ejection of ink draws more ink into the chamber. Such capillary action counteracts the forces of backpressure. Backpressure is the partial vacuum within an inkjet feed slot or local reservoir that resists flow of ink through the printhead. Backpressure is considered in the positive sense so that an increase in backpressure represents an increase in partial vacuum. The backpressure at the printhead at all times is to be strong enough to prevent ink leakage. The backpressure, however, is not to be so strong as to prevent ink droplet ejection.
In a recirculating design, there are other effects in addition to the operational and ambient effects on backpressure. The recirculating flow rate, the pressure drop along the ink pathway, and the volume of ink (in systems including an accumulator) will all affect the backpressure. If the flow rate is too high, there will be a larger pressure drop along the ink pathway, and backpressure at the nozzles may change to an undesirable level. Specifically, it is undesirable for backpressure to increase to a pressure at which the nozzles deprime, or to decrease to the point at which ink leaks out the nozzles. In addition, the nozzles fire optimum droplets at a specific level of backpressure, and printing performance will degrade when backpressure is either too high or too low. Having knowledge of the exact backpressure at the nozzles allows the ink delivery system to adjust flow rate or ink volume to return the backpressure to acceptable levels.
A pen is primed by drawing ink into the nozzle chambers, creating a partial vacuum. This negative pressure, called backpressure, is required to keep ink from leaking out the nozzles. Capillary forces in the printhead nozzles counterbalance the backpressure in the ink, and allow the nozzles to remain full of ink. If backpressure becomes too high (too much vacuum), it will overcome the capillary forces in the nozzles and suck the ink back into the ink reservoir depriming the nozzles. The nozzles lose their ink and become unable to eject droplets. If the backpressure becomes too low (too little vacuum), the ink will spill uncontrollably out of the nozzles and the printhead will be unable to eject controlled droplets.
In prior inkjet printhead designs the backpressure was controlled using an accumulator or pressure regulator to be generally constant, while varying predictably during nozzle firing and nozzle chamber reloading. Foam also has been used to control backpressure forces.
According to the invention, an inkjet pen includes a backpressure sensor integrated into the pen. The pen includes a pen body and an inkjet printhead. The inkjet printhead includes one or more printhead dies. In a preferred embodiment the backpressure sensor is integrated into a printhead die. In an alternative embodiment the backpressure sensor is mounted, fastened or otherwise attached or integrated into the pen body.
According to an aspect of this invention, the backpressure sensor is susceptible to a pressure of ink and to ambient pressure providing an indication of the pressure differential, i.e., the backpressure. For the preferred embodiment the backpressure sensor is susceptible to ink pressure from ink within a printhead ink slot, ink channel or nozzle chamber, and to ambient pressure. For the alternative embodiment, the backpressure sensor is susceptible to ink pressure from ink within an ink slot, ink channel or ink cavity away from a printhead die, and to ambient pressure.
In a preferred embodiment, the backpressure sensor includes piezoresistive strain sensing elements able to respond to differential pressure variations of 0.1-10 inches of water.
According to an advantage of this invention, backpressure is monitored providing an indication of the operation of a printhead. This is beneficial for example in a pen having recirculating ink. The backpressure serves as a feedback for adjusting ink flow rate for a given work load of ink ejection. For example, when ink recirculation is increased to increase a cooling effect on the printhead, the pressure drop from pen inlet to outlet increases. If this pressure drop becomes too great, the pen may deprime or drool. Backpressure provides an indication that can be used to limit the flow rate.
These and other aspects and advantages of the invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings.